In a computer system, printed circuit boards may include conductive paths that electrically connect electronic components, such as chip packages and connectors, mounted to the circuit boards. The conductive paths may include signal traces that extend along the circuit board to carry data signals between the electronic components. In current circuit board designs with many connections between electronic components (e.g., chip-to-chip connections), multi-layer printed circuit boards include multiple signal traces. Signal paths sometimes must travel from a component on the top of the circuit board to the signal traces in the various layers inside of the circuit board.
Access to the inner layers in a multi-layer printed circuit board may be provided by vias, such as plated through-hole vias (PTHs). Vias may be formed by drilling through the circuit board and coating the inner surface with a conductive material. According to existing processes, a hole may be drilled through the entire board, even if a targeted layer (i.e., a signal trace) is located inside the board, leaving an unused portion of the via (referred to as a via stub).
Via stubs may be problematic because they can be a source of resonance in the signal path, which causes extra signal loss. Faster signals with higher frequency content (e.g., greater than about 4 Gb/s or 2 GHz) may have increasing loss due to the resonance in via stubs. Circuit boards having a higher stackup (e.g., in multi-CPU servers) may experience more resonance even at lower frequencies because longer stubs reflect more energy. Multiple stubs within a channel (i.e., between two electronic components) may produce re-reflections causing noise or interference with the signal transmitted by the channel.
Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications, and variations thereof will be apparent to those skilled in the art. Accordingly, it is intended that the claimed subject matter be viewed broadly.